A conventional seat back cushion of a vehicle seat is resiliently supported by a large number of seat springs mounted between side frames of a seat back frame. The seat spring is generally a zigzag spring or a formed wire spring.
There is a known vehicle seat in which a cushion plate is disposed behind the seat back cushion, and the cushion plate is supported by a seat back frame by means of seat springs (Patent Document 1 for example).
There is a known vehicle seat in which the cushion plate is provided with an auxiliary plate which moves backward and forward, and a cushion pressure is changed by the movement of the auxiliary plate (Patent Document 2 for example).    Patent Document 1: JP 2006-021616 A    Patent Document 2: JP 11(1999)-206506A